What is the common difference of an A.P. a21-a7= 84
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Hey genius!
here, given a21 - a7 = 84 .................. ( 1 )
let the first term of the AP be a and the common difference be d.
an = a+ ( n-1) d
a21 = a+ ( 21- 1) d = a + 20d
a7 = a + ( 7 - 1) d= a+ 6d
Substituting the values in equation ( 1 )
a + 20d - ( a + 6d ) = 84
14d = 84
therefore, d= 6 (ans) .
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